MLP is a muscle-restricted cytoskeletal protein that is required to maintain normal cardiomyocyte morphology and function. MLP-deficient mice mimic the morphological and clinical picture of dilated cardiomyopathy and heart failure in humans. MLP has been localized in the Z-bands of the sarcomere where it may bind alpha-actinin, and inside the nucleus where it appears to bind the transcription factor MyoD. The hypothesis of this proposal is that MLP couples external mechanical stimulation with transcriptional events inside the nucleus; that is, MLP is an essential component of a mechanosensor pathway in cardiomyocytes. To address this hypothesis we intend to make a number of point/deletion mutants of the MLP gene. These mutant genes will be used to 1) determine which regions of MLP are important for cellular localization, and 2) to determine what regions of MLP are able to reconstitute stretch response in MLP-deficient cardiomyocytes. We also propose to isolate MLP protein binding proteins. Two strategies will be used: 1) chemical x-linking of MLP and associated proteins from lysates of cardiomyocytes infected with adenovirus expressing hexahistidine tagged MLP, and 2) isolate components from cardiomyocyte lysates that specifically bind to MLP-agarose beads.